THIS weird and wonderful creature is the star-nosed mole (Condylura cristata), a small, semi-aquatic mammal which inhabits the low wetlands of eastern North America. Like other moles, it ekes out an existence in a network of narrow underground tunnels, and digs shallow surface tunnels where it forages for insects, worms and molluscs. Living as it does in almost complete darkness, the star-nosed mole has poorly developed eyes, and is virtually blind. Instead, it relies heavily on its remarkable star-shaped nose. This organ enables the star-nosed mole to decide whether something is edible with astonishing speed – in fact, it recently entered the Guinness Book of Records as the world’s fastest forager – and also to sniff out food underwater.

The star-shaped nose is a highly specialized sensory-motor organ, which consists of 11 pairs of fleshy finger-like appendages, or ‘tendrils’. The star, which is less than half an inch in diameter, is divided into a high resolution central fovea region and less sensitive peripheral areas. It is much larger than the nose of other mole species, covering 0.92 cm2 per touch, compared to 0.11 cm2 covered by the noses of other mole species. The star also contains a far higher density of receptors than the noses of other mole species; its surface is covered with 25,000 mechanoreceptors called Eimer’s organs. (That makes it about 6 times more sensitive than the human hand, which contains about 17,000 receptors.) This makes the star ultrasensitive – it is, in fact, the most sensitive organ in the entire animal kingdom.

The nose is innervated by 100,000 large diameter axons, so that tactile information from it is transmitted to the brain rapidly. Furthermore, the star-nosed mole’s brain processes the information at a very high speed, which approaches the upper limit at which nervous systems are capable of functioning. The mole can therefore decide whether or not something is edible within about 25 milliseconds (ms, or thousandths of a second). Neurons in the mole’s somatosensory cortex – that part of the brain which responds to tactile stimulation – respond to touch within 12 milliseconds, and it is estimated to take a further 5ms for motor commands to be conducted back to the star. By comparison, it takes us humans approximately 600ms to press the brake pedal in response to something that steps out in front of our car.

The star-nosed mole can touch 13 separate areas of the ground every second, and can locate and consume 8 separate prey items in under 2 seconds. When the outer appendages of the star come into contact with a potential food source, the mole moves its nose so that the two lower tendrils, which are the most sensitive, come into contact with it. This pair of tendrils is supplied with more nerve fibres than the others and is therefore far more sensitive. When searching for prey, the mole performs repeated cycles of star movements and touches, typically lasting 50 and 2ms, respectively. Once prey has been identified, it is captured with tweezer-like incisors, whose movements are co-ordinated with those of the star.

The importance of the nose to this organism’s lifestyle is reflected in the way its brain is brain is organized – approximately half of the brain is devoted to processing sensory information from the nose. The nose substitutes for the eyes, with the information from it being processed so as to produce a tactile ‘map’ of the environment under the mole’s nose. Like the somatosensory cortex of other mammals, that of the star-nosed mole is said to be somatotopically organized, such that sensory information from adjacent parts of the nose is processed in adjacent regions of the somatosensory cortex. The tendrils of the nose are therefore “mapped” onto the brain, with the lower, most sensitive pair of tendrils having a larger part of somatosensory cortex devoted to them than the other less sensitive tendrils. And whereas the brains of other mole species contain two sensory maps, that of the star-nosed mole contains three; this may enable it to carry out highly efficient parallel processing of tactile information.

Illustration which graphically depicts the importance of the star-nosed mole’s nose. The appendage is shown at a size that is proportional to the amount of the mole’s brain that is devoted to processing sensory information from it (Laura Finch).

This amazing appendage also enables the mole to smell underwater, something which was previously thought impossible. The animals were filmed with high-speed cameras as they followed underwater scent trails which led to food. They were found to exhale between 8 and 12 small air bubbles per second, each of volume 0.06-0.1 milliltres, onto objects or scent trails they encounter while foraging underwater. The bubbles are then drawn back into the nose, so that odorant molecules in the air bubbles are wafted over the olfactory receptors. When a fine mesh was used to prevent the mole’s exhaled bubbles from coming into contact with the scent trails, the accuracy with which the animals followed the scents dropped to about 50%, confirming that the mole can indeed smell by blowing bubbles, and suggesting that it has to come into contact with, or at least come into close proximity with, a scent trail in order to smell it while underwater.

The star-nosed mole evolved to inhabit a wetland habitat, and so was placed under selective pressure to exploit the dense populations of small insects it found in its new environment. It consumes its prey in large numbers, and the dazzling speed with which it does so counterbalances the low nutritional value of each individual piece of food, and maximizes the time available for finding food. The proximity of the star-shaped nose to the mouth greatly reduces the handling time required before food can be ingested, and is a major factor in how the star-nosed mole can find and eat food so quickly.

Comments

Clare: Yes, the foraging speed is reflex-fast. Each cycle of star movements and touches takes about the same time as the human knee jerk reflex, but the time it takes to determine whether something is edible is about half that.

The diameter of the nerve fibres conducting the impulses is one important factor here – the bigger the diameter of a fibre, the faster it propagates impulses. Conduction velocity is also increased by myelin, the fatty tissue with envelops some nerve fibres.

Another factor is the length of the fibre. Obviously, the longer a fibre is, the more time it will take for an impulse to travel along its length. So, in the case of the knee jerk reflex, the impulses travel from the knee to the spinal cord and back, along myelinated peripheral fibres. This takes approximately 50ms, because it is a simple circuit consisting of just two nerve cells, and does not involve the brain in any way. Conscious reactions, which do involve the brain – such as the example I give in the post – take far longer.

The foraging time is reflex-fast mainly because the distances travelled by the impulses are much shorter. I don’t know the diameter of the fibres involved, but I’d imagine that it’s also some bit bigger than that of the fibres involved in the knee jerk reflex.

When you say “it eeks out an existence”, presumably this is a reference to the sound it must make, as I’m not aware of any other meaning for the onomatopoeic word “eek”, apart perhaps from being a part of a name of a prominent Reggae artist, which itself is a reference to the same original meaning referred to.

I knew the star nosed mole was the freakiest looking mammal in the world, but I didn’t know about all those other claims to fame! I especially liked the somatosensory map graphic (or whatever it’s called!), it’s mostly one giant weirdo tentacled nose.

I was introduced to the star-nosed mole by an article in the July 2002 Scientific American. What I remember best is, as you say, that its brain processes tactile information in a way much like other brains process visual information. Amazing animal.

Adrian: Here’s the article you mention: The nose takes a starring role. It’s by Kenneth Catania of Vanderbilt University, who did all of the work discussed in this post (and who I really should have mentioned).

Agreed. The tactile organ is truly astonishing; it makes me recall the work being done with the mantis shrimp, which is similar in that it has a sense (vision) in which it specializes and does so with absolutly astonishing capacity.

It makes me curious though–the amount of information the mantis shrimp gets from the eyes is such that it must perform on-site (I.e. In the eye) preprocessing. Is the case similar with the mole? How much detemination takes place in the nose?

Agreed. The tactile organ is truly astonishing; it makes me recall the work being done with the mantis shrimp, which is similar in that it has a sense (vision) in which it specializes and does so with absolutly astonishing capacity.

It makes me curious though–the amount of information the mantis shrimp gets from the eyes is such that it must perform on-site (I.e. In the eye) preprocessing. Is the case similar with the mole? How much detemination takes place in the nose?

nick: As far as I know, there’s no processing taking place in the nose itself. Sensory information is sent to the brain, processed there, and a motor output is sent back. This can happen so quickly because the nose is innervated by a huge number of large axons, and because it’s so close to the brain.

Also remember that visual processing involves huge amounts of data, far greater than the tactile sense. So local processing eases the burden on the brain. We know that visual information undergoes some processing in the human retina too, before it is relayed along the optic nerve.

Very cool videos. I wonder if they “whisk” these tendrils the way a rat whisks its whiskers. That is, is there musculature devoted to moving them to and fro even if the head is held fixed, or do they palpate my shaking the head about so that the tendrils touch the object of interest?

Very cool somatosensory molunculus pic. I’m actually surprised by how much cortical coverage is devoted to the back and tail, much more (proportionally speaking) than in the ratunculus, which is basically all snout.